Bimetallic strip for a sliding bearing and process for producing said bimetallic strip

ABSTRACT

A bimetallic strip for a sliding bearing having a sliding strip of an aluminum alloy which is adhered to a steel supporting strip and method of manufacture. The composition of the sliding strip is from 3 to 30% of tin; from 1 to 6% of silicon and the remainder being of aluminum and impurities, and the sliding strip has at least 95% of the silicon hard particles smaller than 3.5 microns and an aluminum grain average size of about 6 microns. The sliding strip is produced by roll casting the alloy and attaching the sliding strip to the steel supporting strip to form the bimetallic strip which is heat treated between 200° and 380° C. to obtain a metallurgical bonding between the strips; subjecting the bimetallic strip to a solubilizing process of the intermetallic compounds of the aluminum alloy by heating at 380-500° C., followed by cooling; and subjecting the bimetallic strip to a precipitation treatment at a temperature from 150° to 250° C. An interlayer can be provided between the sliding strip and support strip.

FIELD OF THE INVENTION

The present invention refers to a bimetallic strip, which is used in theformation of sliding bearings for internal combustion engines and whichcomprises a strip, which is made of an anti-friction or sliding materialin aluminum base alloy usually containing silicon, tin and copper andwhich is adhered to a steel supporting strip. The present invention isalso related to a process for producing said bimetallic strip.

BACKGROUND OF THE INVENTION

The internal combustion engines have been designed to work withincreasingly higher speeds and loads, thereby requiring slidingbearings, which are able to support these increasingly severeroperational conditions, through improved fatigue resistance andanti-sticking characteristics, and which can operate with forged steelor cast iron crankshafts, without the need to protect the bimetallicstrip with a lead/tin electrodeposited overlayer.

It is known that the high fatigue resistance characteristics of thesealuminum base alloys result from a finer and more uniform distributionof the silicon particles in the aluminum phase.

Nevertheless, the known alloys of these type, such as that described inU.S. Pat. No. 4,696,867, still present a silicon particle distributionwhich, in spite of a possible addition of small amounts of strontium orsodium to said alloys, does not reach a desirable refining to obtain aload capacity compatible with the operational requirements of the moderninternal combustion engines.

The grain size of the known alloys of this type is from about 20 to 50μm, avoiding the achievement of substantial increases of fatigueresistance in the produced bimetallic strips (see FIG. 3 in thedrawings). Besides de inconveniences cited above, it should be observedthat the tin percentages of about 8% or more in the alloy composition donot allow said alloy, after its usual casting, to be hot rolled to thedesired thickness for the sliding strip, because hot rolling thesealloys presents the risk of the tin being squeezed out.

Another deficient aspect of the known solutions refers to the lack oftreatment steps of the bimetallic strip, so as to obtain, in the slidingstrip, a compound that is capable of hardening the aluminum alloy,increasing even more its fatigue resistance through a more accentuatedsilicon refining.

These prior art aluminum alloys are cast into billets with a shape whichis very different from that to be attained by the sliding strip, thusmaking difficult and costly the manufacturing process of the bimetallicstrip and the achievement of the minimum silicon refining in the alloystructure.

DISCLOSURE OF THE INVENTION

It is an objective of the present invention to provide a bimetallicstrip for sliding bearings, which comprises an aluminum base alloycontaining silicon and tin and which presents a structure with a highdegree of silicon-tin refining, improved properties of fatigueresistance, anti-sticking and adherence to a supporting strip, withoutrequiring the presence of strontium or sodium in the composition of thealuminum alloy and without leading to risks of the tin being squeezedout during the hot rolling steps of the sliding strip material.

Another objective of the invention is to provide a process which allowsto obtain the above cited bimetallic strip in a economically feasibleway.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below, with reference to the attacheddrawings, in which:

FIG. 1 is a sectional view of a length portion of a bimetallic strip ofthe type used in the present invention;

FIG. 2 is a block diagram, illustrating the steps of the manufacturingprocess of the bimetallic strip shown in FIG. 1;

FIG. 3 is a metallographic representation of the sliding strip structureof a bimetallic strip produced in accordance with the prior art;

FIG. 4 is a metallographic representation similar to that of FIG. 3, butrelative to the sliding strip of the bimetallic strip the presentinvention; and

FIG. 5 is a block diagram, illustrating the steps in the manufacturingprocess of the bimetallic strip including an aluminum interlayer.

BEST MODE FOR CARRYING OUT THE INVENTION

According to a first aspect of the invention with respect to FIG. 1, thebimetallic strip comprises a sliding strip 1, which is made of analuminum base alloy, and which is adhered to a supporting strip 2usually in steel, through a nickel coating layer 3 applied onto thesupporting strip 2.

The aluminum alloy comprises from 3 to 30% of tin; from 1 to 6% ofsilicon and the remaining being of aluminum and accidental impurities.It has the shape of a strip, which is cast by quick solidification, witha thickness sligthly superior to that of the sliding strip 1 to beformed onto the supporting strip 2, said sliding strip 1 beingstructurally solubilized and artificially precipitated, in order topresent an aluminum grain size of about 6 μm. This alloy has the siliconhard particles finely dispersed in the aluminum matrix, as illustratedin FIG. 4, at least 95% of said particles being smaller than 3.5microns.

In a preferred composition, the alloy may further include at least oneof the hardening elements: Ni, Mn, Cr, Cu and Ti, in the range from 0.05to 5%, with the aim of increasing the mechanical properties and thewear, fatigue and sticking resistances.

In case the alloy is provided with one or more of the above additives,at least 95% of the silicon hard particles will be smaller than 3.5microns and 5%, at maximum, will vary from 3.5 to 5 microns.

According to a second aspect of the invention related to FIG. 2, theprocess for producing the above cited composite material comprises thefollowing steps:

a—casting by the quick solidification process (roll cast) an alloycomprising from 3 to 30% of tin, from 1 to 6% of silicon, optionallyfrom 0.05 to 5% of at least one of the hardening additives mentionedabove and the remaining being of aluminum and accidental impurities, bypouring the cast alloy between two cold rolling cylinders.

b—annealing the aluminum alloy sliding strip 1 in a heater at atemperature range from 200 to 500° C.;

c—setting, optionally, the thickness of the sliding strip 1, throughcold rolling passes, followed by respective annealings, as defined inthe previous step;

d—hot rolling (cladding) together the sliding strip 1 and the steelsupporting strip 2 coated with a nickel film 3, in order to form thebimetallic strip;

e—heat treating the bimetallic strip between 200 and 380° C., so as toobtain a metallurgical bonding between the strips;

f—subjecting the bimetallic strip to a solubilizing process of theintermetallic compounds of the aluminum alloy, by sudden heating at380-500° C., followed by sudden cooling; and

g—subjecting the bimetallic strip to an artificial precipitationtreatment in a heater, at a temperature from 150 to 250° C.

The casting step by quick solidification (roll cast) allows theproduction of a sliding strip 1, which has been quickly solidified bythe metallic bath and which has a reduced thickness, in milimeters, thatis already very close to the thickness to be maintained in the finalproduct. Said sliding strip further presents a quite refined structure,mainly in terms of silicon and tin dispersion in the aluminum phase. Therefining of the hard particles, i.e., silicon and silicon compounds,reaches a level in which 100% of the particles are smaller than 3.5microns. Optionally, the alloy may contain the above cited hardeningadditive elements and have the dimension of the hard particlescontrolled, in order to obtain 95% of the particles smaller than 3.5microns and the remaining 5% varying from 3.5 to 5 microns. Thus, it ispossible to initiate the manufacture of a product which is moreresistant to fatigue and sticking than those known products, by using analuminum alloy containing silicon, tin and copper, but without strontiumor sodium, which the prior art considers as adequate for better refiningthe structure of this kind of alloy.

Another unexpected effect in the alloy structure, in function of thecasting process, is that the sliding strip 1 may be hot rolled at hightemperatures, without the tin being squeezed out, even when said tin isfound in porcentages higher than 8%.

The solubilizing step may be achieved, by said bimetallic strip beingsubjected to a liquid bath, such as a lead bath, said bimetallic stripbeing then subjected to sudden cooling through jets of water, forexample. This treatment promotes a substantial increase in the loadcapacity of the bimetallic strip to be produced.

The artificial precipitation is made in heaters, whereto the coils arefed in a discontinuous way.

The bimetallic strip may then be shaped for producing the slidingbearing.

Another unexpected aspect of the invention resides in the fact that theaverage size of the aluminum grain, corresponding to 50 microns in theprior art, is reduced to about 6 microns in the new sliding strip 1,which is obtained by roll cast or hot cladding.

FIG. 5 illustrates a variant of the process, in which there is provideda nickel or aluminum interlayer, onto which a sliding strip is formedand rolled, the latter being obtained by quick solidification.

What is claimed is:
 1. A bimetallic strip for a sliding bearing,comprising a sliding strip of an aluminum alloy which is adhered to asupporting strip of steel wherein the composition of the sliding stripis from about 3 to 30% of tin; from about 1 to 6% of silicon and theremainder being of aluminum and impurities, said sliding strip having atleast 95% of the silicon hard particles smaller than 3.5 microns and analuminum grain average size of about 6 microns.
 2. A bimetallic strip,as in claim 1, wherein the aluminum alloy of the sliding strip furtherincludes, by weight, from 0.05 to 5% of at least one hardening elementselected from the group consisting of Ni, Mn, Cr, Cu and Ti, said alloyhaving at least 95% of the silicon hard particles and hardening additiveelements smaller than 3.5 microns and not more than 5% varying from 3.5to 5 microns.
 3. A bimetallic strip, as in claim 1 further comprising aninterlayer between the sliding strip and the supporting strip. 4.Process for producing a bimetallic strip for a sliding bearingcomprising an aluminum alloy sliding strip adhered to a supporting stripof steel comprising the steps of: a—roll casting a sliding strip of analloy comprising from 3 to 30% of tin, from 1 to 6% of silicon and theremainder of aluminum and impurities to obtain at least 95% of thesilicon particles smaller than 3.5 microns and an aluminum grain averagesize of about 6 microns; b—annealing the aluminum alloy at a temperaturefrom 200 to 500° C.; c—attaching the sliding strip to a steel supportingstrip; d—subjecting the bimetallic strip to a solubilizing process ofthe intermetallic compounds of the aluminum alloy by heating at 380-500°C., followed by cooling; and e—subjecting the bimetallic strip to anartificial precipitation treatment at a temperature from 150 to 250° C.5. Process, as in claim 4, wherein the sliding strip cast in stepa—comprises from 0.05 to 5% of at least one additive hardening elementselected from the group consisting of Ni, Mn, Cr, Cu and Ti.
 6. Process,as in claim 5, wherein the sliding strip has at least 95% of the siliconparticles and additive hardening elements smaller than 3.5 microns andnot more than 5% varying from 3.5 to 5 microns.
 7. Process, as in claim4 wherein the sliding strip is roll cast with an aluminum or nickelstrip interlayer between the sliding strip and the supporting strip. 8.A bimetallic strip as in claim 3 wherein said interlayer is of amaterial selected from the group consisting of aluminum and nickel. 9.Process as in claim 4 further comprising after the annealing step b—thestep of rolling the sliding strip to a desired thickness and annealingthe rolled strip.